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obsidian-full-calendar
Keep events and manage your calendar alongside all your other notes in your Obsidian Vault.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
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Obsidian is a text editor, and I don't think it has a web interface despite being an Electron app, but it's so insanely customizable with plugins that it might be worth a look. I know you can do tasks and Kanban boards, it looks like there is a plug-in for calendars that at least will read entries, it might be possible to put together something very close to what you want there. And your data's always just stored as plain text so you don't have to worry about lock-in.
I've personally gone through the entire process to its inevitable conclusion, starting with plain reminders on my phone, moving to text files (infinite customizability! but also very little functionality!), then 2Do and GoodTask for iOS which both had tons of customization options, and finally culminating in basically making my own todo app in TiddlyWiki. I've got an advantage in that I'm a software engineer already so it was a fun experience and not a frustrating one, and I was able to build up a little ecosystem of scripts and hacks around it to flesh it out and integrate it with the other systems and devices in my life.
Obsidian is a text editor, and I don't think it has a web interface despite being an Electron app, but it's so insanely customizable with plugins that it might be worth a look. I know you can do tasks and Kanban boards, it looks like there is a plug-in for calendars that at least will read entries, it might be possible to put together something very close to what you want there. And your data's always just stored as plain text so you don't have to worry about lock-in.